The embodiments described herein relate generally to combustion systems, and more particularly to combustions systems that use staged fuel combustion.
During a typical combustion process within a furnace or boiler, for example, a flow of combustion gases, or flue gases, is generated. As used herein, the terms “flue gases” and “combustion gases” refer to the products of combustion including, but not limited to, carbon, carbon dioxide, carbon monoxide (CO), water, hydrogen, nitrogen, sulfur dioxide, chlorine, nitrogen oxides (NOX), and/or mercury generated as a result of combusting fuels, such as solid and/or liquid fuels. Combustion gases may contain NOX in the form of a combination of nitric oxide (NO) and nitrogen dioxide (NO2). Various technologies have been applied to combustion systems to minimize the emissions of NOX, however, further improvements are needed.
At least some known furnaces use a staged combustion to reduce the production of at least some of the combustion products, such as nitrogen oxide (NOX). For example, in a three-stage combustion process, fuel and air are combusted in a first stage, fuel in then introduced into the combustion gases in a second stage, and air is then supplied to the combustion gases in a third stage. More specifically, in the second stage, fuel is injected into the combustion gases, without combustion air, sufficient to form a sub-stoichiometric, or fuel rich zone. The term “fuel rich,” as used herein, refers to a condition in which more than a stoichiometric amount of fuel available for reaction with oxygen (O2) present in the available air, i.e., a stoichiometric ratio (SR) of less than about 1.0. The term “fuel lean,” as used herein, refers to a condition in which less than a stoichiometric amount of fuel is available for reaction with oxygen (O2) present in the available air, i.e., an SR of greater than about 1.0. In the second stage, at least some of the fuel combusts to produce hydrocarbon fragments that subsequently react with NOX that may have been produced in the first stage. As such, NOX present in the combustion gases may be reduced to atmospheric nitrogen in the second stage. In the third stage, air is injected to consume the carbon monoxide and unburnt hydrocarbons exiting the second stage. In known systems, the SR within the third stage is greater than approximately 1. Although three-staged combustion systems reduce an amount of NOX in the flue gases exiting the combustion system, further reduction in the amount of NOX in the flue gases is desirable.